(a) Field of the Invention
This invention relates to a silicon carbide process tube used for the heat treatment of semiconductor wafers.
(b) Description of the Prior Art
A process tube is used in the heat treatment such as the diffusion step, oxidation step, etc., included in the manufacturing process of a semiconductor device. It was customary to use a process tube made of quartz glass. However, quartz glass tends to be easily softened, deformed and devitrified under high temperatures, leading to a short life of the process tube made of quartz glass.
To overcome these defects, a silicon carbide process tube has come to be widely used recently. However, a tube of this type is also defective in that a porous SiO.sub.2 film is formed on the inner surface of the tube a certain period after use. The porous film causes dust generation, leading to a low yield of the semiconductor device. To overcome this difficulty, it is necessary to periodically wash the tube. Recently, the dust-free condition is more strongly demanded in order to meet the requirements for an increased degree of integration and in accordance with the trend toward an increased diameter of the wafer, with the result that the process tube must be washed more frequently. However, the increased washing frequency shortens the life of the process tube. In addition, operation of the manufacturing apparatus of the semiconductor device must be stopped because of the laborious washing of the process tube. Naturally, the manufacturing efficiency of the semiconductor device is lowered.